Offshore drilling from floating structures expanded and became commonplace after world war II but the industry has never fully solved the problem caused by rise and fall of the floating structure. The drill ship normally has a derrick above the vessels deck from which a hoist hook suspends most well related payloads. A load suspended from the hook will rise and fall, in sympathy with the heave of the vessel, relative to the sea bed or the bottom of a well. Common rise and fall may be on the order of ten feet. If not compensated for vessel heave, a suspended load may bash sea bed structures and a drill string may damage a drill bit by extreme load variations. The traditional remedy for problems caused by vessel heave has been to suspend operations until the cause of the excess heave subsides. Such suspensions are costly and endanger the well bore.
A vast majority of drilling rigs, sea borne or on land, use a generally common main load handling system. A live cable drum is powered, called the draw works, to drive a wire line extending to a crown block atop the derrick and down to a traveling block, which carries the hook, that moves between the top and bottom of the available travel space. The wire line is reversed about the traveling block by a sheave and returns to the crown block. There is a plurality of sheaves rigged side by side on both the crown and traveling blocks and the wire line makes about six loops about the sets of separated sheaves. The final wrap around the crown block leaves an immobile line, called the dead line, that extends down to the derrick floor where it is clamped off.
Contributing to the bit load control problem is the draw works design feature that provides one operational mode for line payout and another mode for reeling in line. The mode cannot be easily changed while uniform drilling proceeds. Efforts to change that feature have not resulted in significant changes in draw works design. The draw works cannot readily pay out line and immediately take in line. Vessel heave then provides a form of movement that is not within the draw works capability to compensate.
The natural characteristics of water body dynamics pose both problems and potential solutions. Water bodies subject floating structures to three principal forms of surface chance features; wind waves, swells, and tides. During the usual operational weather wind waves are too small to significantly influence large drilling related vessels. Tides are usually so gradual as to fall well within the rate change abilities of automatic drillers. Significant disturbances due to tides are so predictable that simple preparations for the events are possible. Swells cause most heave problems.
Swells individually approach the shape of sine waves and have a cyclic regularity and their amplitude and frequency usually change slowly. Any selected site may be approached by more than one swell system and they usually arrive from different directions. A vessel, then, may be reacting to the resultant coincidence of several swell systems. The vessel rise and fall excursions will usually define a repeated series of events with a repetition pattern, called the period, of only a few minutes. It should be acknowledged that there are locations and conditions, fortunately rare, that can only be described as chaotic in terms of swell behavior. That rare existence should have little impact upon the need presently addressed.
The hook needs two forms of stabilization. First, stabilization is needed to handle risers that extend from sea bed installations, which must be attached to and removed from the installation while suspended from a heaving vessel. That use demands a stable hook relationship to the sea bed during vessel heave and whether or not the load changes. Second, stabilization is needed to handle drill strings that move downward relative to the sea bed while drilling. The bit load needs stabilization during planned downward movement and during vessel response to heave, and bit load may be less than ten percent of the hook load. While drilling, the degree of control can be somewhat relaxed to cut wear and tear. Further, if it is known that no heavy sea is imminent, only part of the compensator capability needs to be kept active.
Efforts to cope with the heave problems have produced two principal forms of compensating apparatus. One form is carried by the traveling block and comprises a power cylinder arrangement with pressure ballast to provide force to equal hook load. The traveling compensator system works but adds to the ton miles of work done by the draw works. A second compensator concept comprises a ballasted support for the crown block which moves it vertically relative to the derrick and allows it to maintain a practically uniform distance between crown block and sea bed. The draw works is mounted on and moving with the heaving vessel and complicates the situation. The greatest drawback, however, is the added structural mass high in the derrick. More is required of a vessel for it to remain within stability limits with the extra weight aloft.
The technical burden of the currently available compensator systems is constant whether they are active or passive, in terms of the dead weight present far above the vessel metacentric height.
There are compatibility problems. When hook load changes are made the vessel changes elevation. Sensors respond to changes and direct controls to make adjustments, which sets off another round of reactions. Two massive systems may be changing and interacting at the same time to different individual natural frequencies. A powered oscillation may be induced that can be destructive and dangerous. As a minimum the compensator system may often be rendered useless.
Some hook loads such as riser assemblies represent payloads greater than currently available heave compensating systems can support. The risers have attached buoyancy features that reduce the weight in water to an amount within the range of the heave compensator rating,. In the presence of heave, however, the acceleration forces are related to inertia, related to the total structural weight of the riser and attached features, regardless of buoyancy provided. The compensator is either nearly perfect in terms of holding the payload static or its rating may be briefly exceeded. Without almost total compensation for heave, handling risers in the vicinity of seabed installations such as well heads presents unacceptable hazards unless near calm conditions prevail.
If full hook load capacity is compensated the related machine and structure weight can be substantial and it can influence vessel behavior. Both location of such machinery and the effect of its actuation has to be considered. It is desirable to place heavy machinery below the vessels metacentric height and available designs do not invite that option. Sudden movement of heavy machine components can result in proportional response of the vessel, which sensors detect, and to which automatic compensators respond to produce further reaction, and a possibly unfortunate feedback loop is established unless there is substantial freedom of choice of location of compensator machinery relative to the vessel.
The search for compensation improvement does not suffer from a shortage of sensor and signal processor availability. Such systems are often called inertial platforms. There are several movement sensor systems available that are quite precise and responsive. They commonly have outputs indicative of acceleration, speed, and distance moved from a reference. The aerospace and defense industries have exceeded the requirements of the drilling environment with proven reliable systems. Outputs from the sensor related signal processor systems can be matched to the controls required to fully deploy the compensators needed. Hydraulic controls are available to satisfy any foreseen need.
Some definitions are in order, primarily dealing with sensors, transducers, and related signal processors. Accelerometers now available can endure the shock related to any anticipated location if they are properly mounted and protected. Processors related to micro computers have no problem delivering velocity and distance moved from a selected reference after a selected time reference. From processors, signals can be directly tailored to drive any control device foreseeable to serve the purpose of the present invention. Feedback devices are sometimes essential to indicate what the controlled system is doing after signals to go into action have been delivered. Safety considerations will dictate use of feedback detector devices to compare evolving actions relative to directions for actions. Such contrivances are in the art and are shown, at least symbolically in this disclosure without details of construction. Of the several sensor and transducer options presented herein, all direct signals to a central processor. This does not preclude distributing the various circuitry sub assemblies among the several defined components.
It is therefore an object of this invention to provide a heave compensation system for a drill ship that will function in the worst expectable sea conditions with machinery that can be altered functionally to involve only the amount of compensation effort that conditions and load demands permit.
It is another object of this invention to provide heave compensation apparatus that provides only the contrivances necessary to utilize existing sensors, sensor signal processors, fluid controls that respond to the sensors to manage the apparatus and to provide means to select combinations of available components to economize the operation for general purpose service.
It is yet another object of this invention to provide a variable capacity wire line storage means, or line reservoir, to operate on the dead line to reciprocate the dead line to influence the position and movement of the traveling block in response to processed signals from movement sensors.
It is still another object of this invention to provide information from periodic heave experience of a vessel to anticipate heave excursions during the next period of vessel movement and to use that anticipated information to avoid hysteresis effects and to move the hook as needed to maintain the hook in a preselected relationship with the earth.
It is yet another object of this invention to use anticipated direction of oncoming heave movement and provide enough power to overcome at least most of the resistance of the hoist related machinery to move in the oncoming direction of motion so that load counterbalance will suffice for compensation in more circumstances.
It is yet another object of this invention to provide heave compensation apparatus that senses movement from the traveling block to compensate for vessel movement and the sum of strain dimensions of the vessel and the related derrick and hoist structures.
It is still another object of this invention to use the dead line for means to locate compensator machinery low in the ship for stability and to orient apparatus such that the movement is least disruptive to the drill ship activity and vessel stability.
It is yet another object of this invention to provide divided line reservoir apparatus so that a sensitive minor portion can be operated to avoid excessive operation of the major component and to provide finer incremental control of load management when needed.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached claims and appended drawings.